Production of silicon-iron sheets having cubic texture



United States Patent Ofiice 3,130,693 Patented Apr. 21, 1964 3,130,093 PRODUCTION OF SILICON JRON SHEETS HAVING CUBIC TEXTURE Dale M. Kohler, lvliddletown, Ohio, assignor to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio No Drawing. Filed Nov. 8, 1960, Ser. No. 67,917

5 Claims. (Cl. 143-111) The invention relates to the manufacture of siliconiron sheet stock having in preponderant degree an orientation which can be designated as (100) [001] by Millers indices, and is commonly called cubic texture. In this orientation cube faces of the grains are parallel to the sheet surfaces, while cube edges are aligned both in the rolling direction and in the transverse direction. It will be evident from this orientation that the material will have a high permeability both in the rolling direction and in Le transverse direction. Whereas other highly oriented silicon-irons, such as those having the (110)[OO1] or cube-on-edge texture, have high permeabilities in the rolling direction but low permeabilities in other directions and therefore if used for the cores of power transformers must be cut into strips and wound upon themselves to insure that the magnetic flux will flow always in the rolling direction, materials having cubic texture do not have this limitation. It is readily possible to make cores from cubic textured sheets by stamping parts of laminations such as L- or E-shaped members.

Materials having the cube-on-edge orientation can be achieved in a high degree by known series of cold rolling operations with intermediate and final anneals, the final anneal generally effecting secondary recrystallization through a grain boundary energy phenomenon. The production of materials having a cubic texture involves substantially difierent problems. In the first place, a stock must be formed which will contain a sufficient number of cubic nuclei. This is much more dificult than the formation of a stock containing a large number of cubeonedge nuclei. It may be pointed out that the larger the number of such cubic nuclei existing prior to a final secondary recrystallization, the smaller will be the grains in the final product, which is advantageous from the standpoint of core loss. It is an object of the present invention to provide a stock which after the final primary recrystallization will contain as large a number of the cubic nuclei as possible.

In the second place, a process must be provided which will develop a satisfactory number of cubic nuclei upon primary recrystallization at a desired final gauge. A process which requires a comparatively large number of cold rolling reduction stages, or which requires as a starting material a previously finished cube-on-edge product, is likely to circumscribe the range of finished gauge which can be obtained or to force the acceptance of materials of lighter gauge than might otherwise be desirable. A process requiring no more than about two cold rolling stages, practiced upon intermediate gauge hot rolled stock or some other suitable starting piece, is highly desirable; and it is an object of this invention to provide such a process.

In the third place, it is an object of the invention to provide a process having the above advantages in which a preponderant cubic orientation can be achieved with certainty in a material having a sufiicient number of cubic nuclei by a secondary recrystallization involving the phenomenon of surface energy.

A principal object of the invention is the provision of a method of manufacturing silicon-iron sheet stock having a very high degree of cubic orientation in a commercial operation which is relatively simple and within the bounds of commercial economy. It is an object of the invention to provide a way of making silicon-iron sheet stock having a superior degree of cubic orientation. It is an object of the invention to achieve this high degree of cubic orientation in a process which requires only two cold rolling stages.

These and other objects of the invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications are accomplished in that method of which an exemplary embodiment will now be described.

It has been indicated that the cubic nuclei in the material which is to be subjected to a secondary recrystallization anneal must either lie in the cubic position or in a position very close to it. This implies, among other things, a high degree of azimuthal orientation, since grains not so aligned are likely to persist in the final product to its detriment.

Researches have shown that if a sheet of silicon-iron at the end of a final stage of cold rolling has a strong component of an orientation which may be designated as (7,10,15)[fi;,10], a primary recrystallization will produce a number of grains in the cubic position to serve as nuclei during a final secondary recrystallization treatment.

When a hot rolled silicon-iron material is cold rolled, a preferred orientation tends to be produced in it. This orientation is designatable as (111)[l12]. To the degree that this orientation is produced, the grains will be improved in azimuthal orientation. Hitherto in the formation of silicon-iron having cubic orientation, the best results have been obtained by starting with a finished product having the cube-on-edge orientation. This is because the crystals will already have a high degree of azimuthal orientation, and also because as a result of the finishing process the metal will be in a pure and clean condition as hereinafter set forth. Such a process is taught in the copending application of Kohler and Littmann, Serial No. 816,889, filed May 29, 1959, entitled Oriented Silicon-Iron and Process of Making t, and assigned to the same assignee. It has the advantages set forth above, but is relatively expensive and requires such an amount of cold rolling as to make it a problem to produce any but light gauge products.

As set forth in another copending application of Kohler and Littmann, Serial No. 819,589, filed June 11, 1959, entitled The Manufacture of Silicon-Iron Having Cubic Texture, and assigned to the same assignee, it is possible to produce from hot-rolled intermediate gauge stock a product having a substantial component of the (7,10,15) [116,10] orientation by two straight-rolling cold reduction stages and an intermediate anneal or primary recrystallization. In such a procedure the initial cold rolling stage produces a (111) [112] orientation, which upon primary recrystallization produces some grains having a modified cube-on-edge or [001] orientation. Some of these grains can be converted into the (7,10,15) [115,10] orientation during the second cold rolling stage. The process facilitates the production of cubic orientation in desirable sheet gauges, but is quite critical in the amounts of cold rolling reduction in the two stages. Generally speaking, it does not produce as many of the cubic nuclei as does the previously outlined process.

The above processes, which can be considered exemplary, produce a (7,10,15)[fi,,10] orientation through one or more intermediate orientation stages. The desirability of the converted product will largely depend upon the degree of azimuthal orientation in it, the efi'iciency of the final secondary recrystallization, and the gauge which may be attained by the necessary manipulative steps. However, it has been found that a material having the ordinary cold rolling orientation, i.e., (111) [112], can be converted to the (7,10,15) [Ti 5,10] orientation directly by cross-rolling as hereinafter set forth. Such material, subjected to primary recrystallization, will produce a very large number of nuclei which can serve as cubic nuclei in a secondary recrystallization.

It has hitherto been suggested that silicon-iron intended for preferred orientation might be cross-rolled. In the formation of cube-on-edge materials the secondary recrystallization proceeds by grain boundary energy, as has been indicated. Cross-rolling is neither necessary nor desirable in the formation of cube-on-edge orientations; but attempts to produce cubic orientation by cold rolling treatrnents involving cross-rolling followed by a grain boundary energy recrystallization have not been fully successful although the chemistry of the metal has been modified, notably by the addition of aluminum. While the basic phenomena have apparently not been understood by the practitioners in the art, it is believed that in a grain boundary energy recrystallization the lowest energy crystalline form is the cube-on-edge and that this situation cannot be adequately corrected by changes in the chemistry of the metal. Further, the metal of altered chemistry is not as advantageous as is the metal hereinafter described.

By silicon-iron is meant a ferrous material containing sufiicient silicon to do away with phase changes. Ordinarily the silicon may range from 2.5% to 3.5% or higher although the material becomes more diflicult to work if the amount of silicon substantially exceeds about 3.5%. A silicon content of about 3% is preferred. The material will ordinarily contain from about .06% to .12% manganese and not more than about .025 sulfur. The carbon content should not exceed about .04%. The remainder of the material will be iron with only such impurities as are of trace character and are inherent in the manufacture of the iron. In particular the alloys should be free of aluminum or any other element which would interfere with secondary recrystallization of the type in which surface energy causes the growth of the cubic grains. Since the material is to be subjected to secondary recrystallization, it should be clean, and in particular it should not contain more than .015 total oxide. This total oxide content can be further reduced during the processing, and just before the secondary recrystallization the total oxide content will preferably not exceed .006%.

The silicon-iron can be made in any of the known ways; and it is an advantage of this invention that vacuum melting, although available, is not necessary. The metal may be refined in the open hearth furnace or by any of those methods involving at certain stages blowing with oxygen. The metal will be a killed steel; and the silicon content will ordinarily be controlled by the addition of ferro-silicon in the ladle or in a runner.

V The metal may be cast into ingots which are then heated in soaking pits, reduced on a blooming mill, and hot rolled into slabs. The slabs may be reheated and rolled to the desired intermediate gauge on a continuous hot rolling mill. Or a practice may be utilized in which the ingots are continuously rolled to the intermediate gauge material without reheating.

It is possible, as by the use of suitable molds or in a continuous casting machine, to form silicon-iron having a thin section. If the material as cast has a thickness not greater than about .1 to .2 inch, hot rolling may be dispensed with. However, in normal operation, the material will usually be hot rolled to an intermediate gauge of about .1 inch. It may be given a box anneal at this point if desired. If the box anneal is conducted in the presence of the hot mill scale, substantial decarburization will result. In any event the hot rolled product will be prepared by pickling or otherwise for the subsequent cold rolling.

It is desirable that the material just prior to the secondary recrystallization shall contain no more than about .006% carbon. Consequently, if the material has not already been decarburized or if further decarburization is desired, the primary recrystallization anneal hereinafter described may be a decarburizing anneal in a continuous furnace in a wet hydrogen-bearing atmosphere in accordance with the teachings of the Carpenter et al. United States Patent 2,287,476, issued June 23, 1942.

The initial cold rolling is a reduction of about 40% to (60% being preferred). This may be carried on rapidly in a continuous cold mill with the material in strip form and of indefinite length.

Following the first intermediate anneal, the metal is decoiled and cut apart into sheets suitable for crossrolling. The cross-rolling is carried on in a suitable cold mill with a reduction of about 40% to 70% (50% being preferred).

It may be noted that there is no recrystallizing anneal between the straight-rolling and cross-rolling steps just described. Grains having the (ll1)[1l2] orientation as a result of straight-rolling are tilted in part at least to the (7,10,15 1 3, ,l0] orientation by the cross-rolling, so that the cold rolled sheet will now have a strong component of the latter orientation. Upon primary recrystallization it will develop a satisfactory number of cubic nuclei.

It may be given a separate primary recrystallization treatment at a temperature of about 1300" to 1800" F. if desired; and if further decarburization or reduction of oxide inclusions is desired at this point, the primary recrystallization will be a separate heat treatment. However, the secondary recrystallization, which is the last step in the process, involves heating the metal to a still higher temperature, namely about 2200 F.; and it will be evident that the primary recrystallization can occur as the metal is being heated up to the temperature for secondary recrystallization.

It will be observed that the process involves two stages of cold rolling. With two stages of cold rolling it is readily possible to reduce the material from a reasonable hot rolled gauge to a final gauge within the range of sheet thicknesses ordinarily desirable for magnetic steels. This is difiicult to do with processes including a larger number of cold rolling stages and is practically impossible where the starting material is a finished oriented product already of sheet gauge.- A thickness of .012 inch is a typical but non-limiting finished sheet gauge, and is easily attained with a hot rolled starting material about .1 inch in thickness.

The secondary recrystallization treatment is a treatment in which the cubic grains grow by surface energy. In the preferred form of the treatment the material is heated in an atmosphere of hydrogen containing a minute trace of a polar compound such as hydrogen sulfide or carbon monoxide. Such a treatment is generally described in the copending application of Kohler and Jackson, Serial No. 813,289, filed May 14, 1959, and entitled The Production of Oriented Silicon-Iron Sheets by Secondary Recrystallization, and assigned to the same assignee. As explained in that application the presence of the polar compound in the annealing atmosphere shifts the surface energy of the crystals so that crystals having the cubic orientation become the low energy crystals and tend to grow at the expense of crystals having a different orientation. The chemistry of the metal should be such as to promote surface energy phenomena and minimize grain boundary energy phenomena, i.e., the metal should be both pure and clean as defined above.

The cast material was hot rolled to a thickness of .060 inch, cold reduced to .025 inch by straight rolling, cold reduced to .012 inch by cross-rolling, and annealed at 2200 F. for eight hours in a hydrogen atmosphere containing 50 p.p.m. of hydrogen sulfide. The resulting material had a substantially 100% cubic texture.

Other material processed similarly except that the atmosphere of the final anneal did not contain a polar compound showed a very small number of grains having a cubic texture.

Modifications may be made in the invention without departing from the spirit of it. The invention having been described in an exemplary embodiment, what is claimed as new and desired to be secured by Letters Patent is:

1. A process of producing silicon-iron sheet stock characterized by a crystal orientation in which the majority of the grains have their lattices oriented such that the cube faces are substantially parallel to the sheet surface and the cube edges are substantially aligned in the rolling directions and in a direction perpendicular thereto, which comprises providing an elongated strip of hot formed siliconiron stock of intermediate gauge consisting of about 2.5% to 3.5% silicon, about .06% to .12% manganese, not more than about .025% sulfur, not more than about .04% carbon, not more than about .015 total oxides, and substantially free of aluminum, the balance being iron and incidental impurities, subjecting said stock to a first cold rolling reduction in the direction of its length of about 40% to then cutting apart the strip into sheet lengths and cold rolling said sheet stock 40% to 70% in a direction to the first rolling direction in the absence of an anneal between the two cold rolling reductions, heat treating said cross-rolled stock at a temperature of about 1300 to 1800 F. for primary recrystallization and to a surface energy secondary recrystallization at a higher temperature on the order of 2200 F.

2. The process claimed in claim 1 in which the surface energy secondary recrystallization is conducted in an atmosphere of hydrogen containing a minute quantity of a polar compound.

3. The process claimed in claim 2 wherein the polar compound is hydrogen sulfide.

4. The process claimed in claim 1 wherein the intermediate gauge stock is a hot rolled product having a thickness in the range of about 0.1 to 0.5 inch, and wherein the hot rolled intermediate gauge stock is given an annealing prior to the first cold rolling reduction.

5. The process claimed in claim 1 wherein the interme diate gauge stock is a cast structure having an initial thickness not greater than about 0.1 to 0.2 inch.

References Cited in the file of this patent UNITED STATES PATENTS 2,173,240 Wassermann Sept. 19, 1939 2,599,340 Littmann June 3, 1952 2,867,557 Crede et a1 Jan. 6, 1959 2,992,951 Aspden July 18, 1961 2,992,952 Assmus et a1. July 18, 1961 3,008,856 Mobius Nov. 14, 1961 3,061,486 Jackson Oct. 30, 1962 FOREIGN PATENTS 576,774 Belgium July 16, 1959 1,192,271 France Apr. 20, 1959 1,009,214 Germany May 29, 1957 OTHER REFERENCES Journal of Applied Physics, Supplement to v. 31, No. 5, May 1960 (pages 408S-409S relied upon). 

1. A PROCESS OF PRODUCING SILICON-IRON SHEET STOCK CHARACTERIZED BY A CRYSTAL ORIENTATION IN WHICH THE MAJORITY OF THE GRAINS HAVE THEIR LATTICES ORIENTED SUCH THAT THE CUBE FACES ARE SUBSTANTIALLY PARALLEL TO THE SHEET SURFACE AND THE CUBE EDGES ARE SUBSTANTIALLY ALIGNED IN THE ROLLING DIRECTIONS AND IN A DIRECTION PERPENDICULAR THERETO, WHICH COMPRISES PROVIDING AN ELONGATED STRIP OF HOT FORMED SILICONIRON STOCK OF INTERMEDIATE GAUGE CONSISTING OF ABOUT 2.5% TO 3.5% SILICON, ABOUT .06% TO .12% MANGANESE, NOT MORE THAN ABOUT .025% SULFUR, NOT MORE THAN ABOUT .04% CARBON, NOT MORE THAN ABOUT .015% TOTAL OXIDES, AND SUBSTANTIALLY FREE OF ALUMINUM, THE BALANCE BEING IRON AND INCIDENTAL IMPURITIES, SUBJECTING SAID STOCK TO A FIRST COLD ROLLING REDUCTION IN THE DIRECTION OF ITS LENGTH OF ABOUT 40% TO 85%, THEN CUTTING APART THE STRIP INTO SHEET LENGTHS AND COLD ROLLING SAID SHEET STOCK 40% TO 70% IN A DIRECTION 90* TO THE FIRST ROLLING DIRECTION IN THE ABSENCE OF AN ANNEAL BETWEEN THE TWO COLD ROLLING REDUCTIONS, HEAT TREATING SAID CROSS-ROLLED STOCK AT A TEMPERATURE OF ABOUT 1300* TO 1800*F. FOR PRIMARY RECRYSTALLIZATION AND TO A SURFACE ENERGY SECONDARY RECRYSTALLIZATION AT A HIGHER TEMPERATURE ON THE ORDER OF 2200*F. 